Electrodischarge machining (EDM) is a common technique used, for example, for drilling holes in turbine airfoils of a gas turbine engine. EDM drilling essentially involves placing an electrode in a spaced relationship with the workpiece from which it is desired to erode metal by erosive electrical intermittent discharge. The pulsating discharges erode metal until a hole of desired dimensions is formed, after which time the electrode is withdrawn from the proximity of the workpiece.
One of the problems encountered in EDM drilling is achieving proper alignment of the workpiece with the electrode. Difficulties in alignment may result, for example, from the workpiece not having a good datum surface, or from the EDM machine not having any datum relation to the workpiece at all. In the case of portable EDM drilling systems, a workpiece typically is not placed on an EDM worktable, but rather on a separate fixture that is not connected to the EDM machine. Operators generally must rely on the use of simple gauges or visual observation to carry out alignment by trial-and-error. Often the workpiece itself is expensive, which makes alignment by trial-and-error wholly unacceptable, as inaccurate drilling can cause irreparable damage to the workpiece.
There remains a need for a more efficient and accurate method for aligning electrodes in EDM drilling.
The present invention is directed to a method of aligning an electrode with a workpiece in at least two planes in a multiple-axis electrodischarge machining (EDM) drill. The EDM drill comprises at least two rotary axes and a chuck for receiving a first end of the electrode. The electrode has a longitudinal axis and a distal end extending a predetermined distance from the rotary center of the rotary axis of the EDM drill.
In a preferred embodiment of the invention, the electrode is aligned with the workpiece using at least four points of reference on the workpiece. The method comprises placing the workpiece in approximate visual alignment relative to the electrode positioned along a first axis. The electrode then is positioned at a first angle, in a first plane and in a first direction relative to the first axis. A first distance between the rotary center of the rotary axis and a first point on the workpiece co-linear with the longitudinal axis of the so-positioned electrode is determined.
The electrode next is positioned at the first angle, in the first plane, but in the opposite direction relative to the first axis. A second distance between the rotary center of the rotary axis and a second point on the workpiece co-linear with the longitudinal axis of the so-positioned electrode is determined. By comparing the first distance to the second distance, it can be determined whether the electrode is aligned with the workpiece in the first plane. For example, when the intended position of the electrode is perpendicular to the workpiece, if the first distance and the second distance are equal, then the electrode is properly aligned in the first plane. If the electrode is not aligned, an angle of rotation needed to align the electrode with the workpiece in the first plane can be determined empirically.
The electrode then is aligned in a second perpendicular plane in an analogous manner. The electrode is positioned at a second angle, in the second plane and in a third direction relative to the first axis. A third distance between the rotary center of the rotary axis of the EDM drill and a third point on the workpiece co-linear with the longitudinal axis of the so-positioned electrode is determined.
The electrode thereafter is positioned at the second angle, in the second plane, but in the opposite direction relative to the first axis. A fourth distance between the rotary center of the rotary axis and a fourth point on the workpiece co-linear with the longitudinal axis of the so-positioned electrode is determined. By comparing the third distance to the fourth distance, it can be determined whether the electrode is aligned with the workpiece in the second plane. For example, when the intended position of the electrode is perpendicular to the workpiece, if the third distance and the fourth distance are equal, then the electrode is properly aligned in the second plane. If the electrode is not aligned, an angle of rotation needed to align the electrode with the workpiece in the second plane can be determined empirically.
The distance between the rotary center of the rotary axis and the various points on the workpiece co-linear with the longitudinal axis of the electrode can be determined using any suitable technique. One preferred way is to displace the electrode along its longitudinal axis into contact with the workpiece. The distance of displacement can be measured, e.g., by the EDM machine itself, and simply added to the fixed distance from the rotary center of the rotary axis to the distal end of the electrode.
If alignment is needed, the EDM machine (or an auxiliary computer, for example) can calculate and display the amount and direction of rotation needed for correcting alignment in each plane. Alternatively, the EDM machine can be adapted to automatically adjust the position of the electrode in response to the calculations.
In an alternative embodiment of the invention, an electrode is aligned with a workpiece in at least two planes using three reference points on the workpiece surface. The three reference points on the workpiece surface relative to the rotary center of the rotary axis are determined in a similar manner as previously described. These reference points then are used to mathematically determine the normal directions of the workpiece surface. Any necessary axis adjustments relative to the normal directions can be determined empirically.
The present invention provides an efficient and accurate method for aligning electrodes in multiple-axis EDM drilling. The alignment method of the present invention greatly reduces the risk of damage to workpieces during EDM drilling as a result of poor alignment, which is particularly important when expensive workpieces are drilled.